I use Nissens part number 70220 for vehicles that I convert. It is like the P38 matrix - in that it has plastic end tanks which are crimped to the metal heater core part (like the radiator has - except tanks are top/bottom)
Instead of having the metal pipes through the firewall, you replace these with a couple of bits of standard coolant hose (if using the 70220 then the outlet fit 19mm hose nicely - which is the same as the pipework in the P38 engine bay, so no additional reducers etc) and some clamps of your choice.
I had to tweak my clamps up after a month or so as things settled in, but with a bit of thought on clamp positioning, you can get it so you only need to take the footwell trim and the drivers knee panel off to get in to tighten them up.
Ultimately it's up to you - I went with the heater core swap as when I went in to do my O-rings when I first bought the vehicle, I pulled out nearly new O-rings, and found the plastic lip on the heater core itself was damaged - so figured I'd swap it when it came out and then not have O-ring issues ever again...